The increasing number of resources available through portals establish a need to tailor information to individual needs and situations. Mashups are tools for dynamically integrating independent applications. For portals, what is needed are means to automatically create personalized mashups that optimally fit a user's information needs in a given situation. At the core of our approach are different ontology-based models that describe the user, the domain, possible information needs in this domain, and personalization rules determining which information to present to which user in which situation.